There are different classifications for forging. According to the classification based on temperature, forging is classified as cold forging, semi hot/warm forging and hot forging. Forging die type classification includes open die forging and closed die forging.
In hot forging and warm/semi-hot forging, the workpiece material is heated. Therefore forging dies before the forging process must be heated to avoid thermal shocks.
Although the workpiece material is initially at the room temperature in cold forging, heat is generated during the forging process due to the deformation of the workpiece. Therefore, cold forging dies may also be required to be heated before the forging process to avoid thermal shocks.
Improper heating of forging dies results in a variety of problems. The most significant one is the short die life, which is observed as a result of early failure or distortion due to thermal fatigue and non-uniform temperature distribution throughout the surface of the forging die.
Die Heating in Forging Process
The direct gas flame heating and the furnace heating are external die heating methods used in forging process. The first one is the most commonly used method in industry.
During the direct gas flame heating of the forging die, the forging equipment is generally kept idle. The problems encountered during the gas flame torch heating are mainly long heating time and uncontrollable temperature distribution. To keep the forging dies at the required temperature during production of a batch of forging, the forging dies are reheated frequently by means of the gas flame torches by stopping the process from time to time. This type of heating requires considerable shop floor time. The use of gas flame
heating method is a very inefficient and unsatisfactory method of heating because only a small portion of the generated heat of the combustion gasses is transferred to the forging die while most of the heat escapes to air.
In furnace heating, the forging dies are placed in to the furnace before the heated dies are located on the forging press. Although, uniform temperature may be obtained throughout the forging die. One of the main disadvantages of the furnace heating in forging industry is that the forging dies need to be assembled and disassembled each time when heating is required. The other disadvantage is the limitation of the furnace size. These facts cause an increased process time and cost.
In US 2011/02509074 A1, a die holder with electrical resistance cartridge heaters is used to heat the dies in forging process. The die holder is used to hold the die in forging press. The die holder has also temperature sensors for monitoring temperature distribution on the die. To reduce the press forces applied to resistance heaters and temperature sensors on the die holder, the resistance heaters are located in the middle of the die holders. This can reduce the foregoing press load placed on the heaters but the distance from the dies is high and the heat generated from the heaters is poorly transmitted to the dies and the heating time increases. As a result, a problem arises that the heaters cannot efficiently heat the dies to the required temperature ranges for preheating and keeping the die hot.
In U.S. Pat. No. 3,783,669, the structure under the base of the forging die provides gas heating. The structure is made up of multiple bars of high strength materials located between the dies and the anvil of the forging press. The bearing bars take the forging loads. Insulation and burners that are located in bearing bars receive no forging loads. This method is an inefficient method to heat the dies and the combustion gasses pollutes environment.
US 2010/0307216 A1, U.S. Pat. No. 3,893,318, U.S. Pat. No. 4,889,570, U.S. Pat. No. 6,960,746 and U.S. Pat. No. 4,088,000 can also be recognized as prior art techniques for die heating, which none of them concerns of internal heating of dies.
Possible Applicability in Forging Industry of the Die Heating Methods Used for Other Manufacturing Processes
There are some methods used in industry to heat the dies of the manufacturing processes other than forging. The electrical and gas radiant heating use radiation as the heat transfer mean. Heating temperature is available instantly. Radiation to atmosphere is the main loss of the energy. The internal regions of the large dies need long preheating time to reach desired temperature. If the heater is placed too close to the die, this negatively affects condition of the heater itself by increasing temperature of the heater. This may cause damage of the heater due to excessive heating. Gas radiant heating method also creates pollution problem.
The electrically heated air or gas plasma torch provides air heated to 750° C.-1300° C. without air or noise pollution. The use of such system reduces the power required to heat the dies compared to the furnace heating and better temperature distribution compared to gas flame heating. During heating by air, a heat insulation blanket is applied to get better heating performance and reduce heat losses. During forging process, it is not possible to use air heating method with insulation blankets.
The technical disadvantages and problems of the prior art techniques for forging die heating can be lined up as;                Operational difficulty.        Long heating time,        Operational time losses,        Energy losses,        Causing environmental pollution due to combustion gasses.        Decarburization on forging die surface due to gas flames.        Non-uniform temperature distribution on forging dies,        High die surface wear that results in increasing the number of rework of the forging dies,        Undesired rapid cooling of forging dies,        Poor part quality due to forging die wear,        Reduce batch size,        Risk of broken or damaged forging dies,        Thermal fatigue on forging die surfaces,        